Exactly. Because some business requirements require sticking to established standards instead of a wink and a promise. You can't archive documents and expect to be able to read them in 100 years if the "standard" keeps changing every software release. So if Microsoft can't support the standard (and it can't -- because it must maintain some kind of lock-in leverage), it doesn't meet requirements, and some people will look elsewhere.
Yeah, because standards never change. But so what, OSS doesn't guarantee open standards, all it is is open source. There is a HUGE difference.